The present invention refers to a packaging container which is flat when being empty for receiving bulk material and is of a type having an interior and a moisture-proof thermoplastic layer provided with a ventilation zone extending parallel at a distance to the longitudinal edges. Further, the present invention refers to a method of making an empty packaging container as well as a method and apparatus for ventilating and sealing the filled packaging container.
In order to raise the quality assurance of their contents, packaging containers, such as open sacks or sacks with bottom, should protect the contents from exposure to moisture penetrating through the sack layers. Conventional packaging containers come in various designs and are made of a plastic sheet, e.g. of a seamless tube or a sheet web which is subsequently formed to a tube. Further, paper sacks are known which usually include at least two paper layers, with a thermoplastic moisture-proof layer, e.g. a plastic liner being inserted between the paper layer for preventing a penetration of moisture. Paper sacks are exclusively made out of paper webs to form a continuous tube from which tube sections are cut to make the respective sacks.
Although a penetration of moisture is significantly decreased in paper sacks containing a thermoplastic moisture-proof layer or in sacks which are completely made of plastic material, there is the drawback that the sack interior cannot be ventilated. Air usually enters the sack interior during a filling process, especially in connection with powdery material when air is added to the material to render it transportable. Even though more recent filling methods allow a reduction of supply of air, a complete omission is not yet possible. Proposals to suitably design the filling machine in order to allow a withdrawal of air during the filling process remained unsatisfactory because e.g. in open sacks, residual air will still remain in the area of the filling unit.
In connection with filling coarse or grainy material, the ventilation during the filling process is satisfactory since a withdrawal of air is not inhibited by these contents. This is even true when closing the valve immediately after removal of the valve sacks from the feed socket of the filling machine. In sacks which are open at the top, residual air can be separated through pressure application above the contents before the closing step and then withdrawn through vacuum. While the ventilation is generally sufficient during filling of grainy or coarse material, the results during filling of powdery material remain unsatisfactory.
It is also known in connection with plastic sacks which contain granular material to puncture the side wall with a needle and then to evacuate the sack interior. Once ventilation is accomplished, the pierced location is covered by a sealing tape. This method is suitable only for granular material. Moreover, the sealing tape is made of a different material than the sack so that a subsequent disposal thereof causes problems because a reuse of the packaging material requires that the package is made of same or similar material.
It was further proposed to prevent a penetration of moisture into powdery contents by disposing an air tight thermoplastic intermediate layer between the paper layers. While this solution is suitable to protect the contents from penetration of moisture, it significantly complicates a ventilation of the contents. Therefore, proposals were made to leave the overlapping border strips of this air tight layer open. In this manner, a ventilation during filling and transport of the filled sack from the filling machine to the first peripheral device is possible since paper is air permeable and acts like a filter. Although a sufficient ventilation is ensured, this solution has the drawback that moisture can penetrate through the open longitudinal seam of the intermediate layer.
In connection with single-layer plastic sacks, it is also known to place a filter-like strip of non-woven fabric in the area of the longitudinal seam. This proposal also does not sufficiently inhibit a penetration of moisture through the non-woven strip into the sack content.